I grant Emma may have nodded a greeting to Miss Bates without any evidence but for more than that I need at least a tiny hint to latch onto. Emma’s idea the party is so large “it was not necessary” (26) to approach Jane is not taken up by Mrs. Weston or Frank who attend her and Miss Bates. Mrs. Weston poses an opposite view: “This is the luxury of a large party… one can get near every body, and say every thing” (26). Emma’s view is ungracious in comparison. Mrs. Weston takes the opportunity to attend many friends while Emma settles herself onto a chair to be attended upon with the knowledge her lack of attention will not be obvious among so many.

IMO it is the same old story. Emma keeps to the letter of propriety but goes no further if the prospect of it is disagreeable. If a large party makes it acceptable to ignore an old family friend why does Emma rationalize her decision? Her wish to spare Jane an unfair curiosity is generous on the face of it but it is also a convenient excuse to follow her own inclination. It’s an excuse because if Emma really felt for Jane she could have done something to relieve her but instead she sits on her throne amused by her friend’s labor and Jane’s entrapment:

Mrs. Weston, kind-hearted and musical, was particularly interested by the circumstance, and Emma could not help being amused at her perseverance in dwelling on the subject; and having so much to ask and to say as to tone, touch, and pedal, totally unsuspicious of that wish of saying as little about it as possible, which she plainly read in the fair heroine's countenance. (26)

I can’t help being reminded of Emma’s amusement at her own persistent questioning of Jane to satisfy another unfair curiosity in Ch. 20. When Emma gives a party for the Eltons she gets a taste of her own medicine in her own drawing room when the guest of honor holds her own court slighting Emma and Mrs. Weston. It is too parallel a situation for me to consider Emma’s slighting Jane and Miss Bates at the Coles party was acceptable while Mrs. Elton’s slighting her and Miss Weston is not:

When the ladies returned to the drawing-room after dinner, Emma found it hardly possible to prevent their making two distinct parties; -- with so much perseverance in judging and behaving ill did Mrs. Elton engross Jane Fairfax and slight herself. She and Mrs. Weston were obliged to be almost always either talking together or silent together. Mrs. Elton left them no choice. If Jane repressed her for a little time, she soon began again… (35)

I only expect Emma to do what she ought. In Ch. 32 Emma tries to talk her father out of his regret for not waiting on Mr. and Mrs. Elton on the occasion of their marriage. He said: “I would always wish to pay every proper attention to a lady -- and a bride, especially, is never to be neglected” (32). Emma suggests he need not be so anxious to encourage people to marry because he is no friend to matrimony but Mr. Woodhouse will not give way to her view that his personal opinion of marriage alleviates his duty to his friends. Emma thinks he does not understand her but I think it is Emma who is remiss. She is, as she often does, letting her personal feelings about a person cloud her judgment about what ought to be done:

"My dear, you do not understand me. This is a matter of mere common politeness and good-breeding, and has nothing to do with any encouragement to people to marry."

Emma had done. Her father was growing nervous, and could not understand her. (32)